More specifically, the invention relates to a method of assisting an operator piloting an aircraft operated by an organization in a choice of rerouting airports by the operator. The operator should preferably land the aircraft at an airport belonging to the network of the organization. Since a rerouting generates delays for the passengers of the aircraft and additional costs for the organization, the rerouting of the aircraft to an airport in which the organization has a commercial service will have an economic impact that it will more easily be able to absorb. In this document, the terms “airline company” or “company” are used to denote an organization that operates an aircraft.
An aircraft is normally equipped with a flight management system FMS which helps the operator, particularly in generating a flight plan and during the cruising phase.
The FMS can access a navigation database, or NAVDB, which contains aeronautical information concerning a wide geographic zone, for example Europe, in particular information on the airports and the runways of the airports (orientation, length, etc.). The navigation database NAVDB contains an AIRPORTS table containing the names of all the airports in a given geographic zone.
Normally, the company that operates the aircraft equips the aircraft with a CoRoute database which contains routes linking predefined airports from the airport names in the AIRPORTS table. These routes contain information on the air corridors to be taken by the aircraft to link two given airports. The information in the CoRoute database facilitates the programming of the flight plans. This CoRoute database is bought by the company from an aeronautical database supplier. The price of the CoRoute base is linked to the number of routes that it contains. Consequently, a CoRoute database bought by an airline company preferably contains routes that exclusively link airports that are served regularly by the company and in which it has a technical or commercial establishment. The establishment is either wholly owned by the company or is made available by a company allied under the terms of cross agreements (this latter situation is known as a “code-share” situation).
Before a flight, the operator enters into the FMS the name of the airport of departure and the name of the destination airport that the aircraft is required to reach. From information contained in the CoRoute database, and by taking into account meteorological information, the FMS establishes a nominal flight plan to be followed by the aircraft to link the airport of departure to the destination airport. The information contained in the CoRoute database helps facilitate the creation of the flight plan.
Once the destination airport has been selected and the nominal flight plan created, the operator must also, by regulations, programme a flight plan linking the destination airport to an alternate airport.
To do this, the FMS first accesses an ALTERNATE DESTINATION database which contains a list made up of the names of alternate airports related to the destination airport. Then, the FMS automatically generates a flight plan to link the destination airport and the alternate airport chosen by the operator.
The ALTERNATE DESTINATION database associates, with an airport in the AIRPORTS table, a list of pre-established alternate airport names. The creation of this list is based on a distance criterion between the geographic position of the destination airport and the geographic position of the alternate airport. The distance criterion uses a “ground distance”, that is, a distance to be travelled over the Earth to reach the geographic position of the destination airport from the geographic position of the alternate airport.
Normally, an airport Ad is included in a list of alternate airports associated with a destination airport AID, if it has a geographic position that satisfies two constraints:                on the one hand, the ground distance separating the alternate airport Ad from the destination airport AID must be reduced for the duration of the rerouting to be as short as possible and then for the rerouting not to incur too great a delay for the passengers of the rerouted aircraft;        on the other hand, the ground distance separating the alternate airport Ad from the destination airport AID must be great enough for the alternate airport to be subject to weather conditions that are substantially different from those to which the destination airport is subject.        
For example, the names of the Amsterdam or Brussels airports can be associated with the destination airport name “Roissy-Charles-de-Gaulle” in an ALTERNATE DESTINATION database.
The ALTERNATE DESTINATION database is also bought by the airline companies from aeronautical database suppliers. Here too, the price of the database increases with the number of destination airports AID that it contains. Consequently, an ALTERNATE DESTINATION database bought by an airline company preferably contains names of airports that the company chooses as relevant destination. That is, the airport names included in the ALTERNATE DESTINATION table are normally regularly served by the company and probably it has a technical or commercial establishment there, wholly owned or on a code-share basis.
Moreover, during the cruising phase, when the operator of the aircraft unexpectedly wants to modify the airport that the aircraft must reach, for example after discovering a failure of a critical device on the aircraft or the occurrence of a severe climatic event in the vicinity of the destination airport. Since the alternate airport programmed into the flight plan is chosen taking into account exclusively the position of the destination airport and not that of the aircraft at the moment when the rerouting is requested, the flight management system FMS proposes to the operator, at his request, a list of rerouting airports enabling him to land as early as possible. This list of rerouting airports is generated from the content of the AIRPORTS database.
One drawback of the list of rerouting airport names compiled from the AIRPORTS database arises from the fact that the content of the list is determined without taking into account the frequency with which the airports are served by the company. In other words, the content of the list does not take account of whether airports belong to the company's network.
Now, this information is important, because it conditions the cost of the rerouting for the company: in practice, for logistical reasons, this cost is higher when a rerouting takes place to an airport in which the company is not present than when the rerouting takes place to an airport in which the company has a technical or commercial representation. Still for logistical reasons, the delays incurred for the passengers of the rerouted aircraft are normally shorter when the rerouting takes place to an airport in which the company has a representation. For these two reasons, it would be good to prioritize such airports at the moment when a rerouting is envisaged.
Moreover, another drawback is encountered in certain particular situations, for example when an aircraft rerouting takes place above a continental zone which is densely populated with airports, typically Europe or the United States of America. In such a situation, there are simultaneously a large number of airports that are separated from the aircraft by a distance which is less than an immediate descent distance. The immediate descent distance is a minimum distance to be travelled by the aircraft to reach the ground. It is equal, typically, for a hundred-seat aircraft in the cruising phase, to the distance travelled by the aircraft during twenty minutes of flight and, in a zone densely populated with airports, it is exclusively a function of the altitude of the aircraft and of the elevation of the airport to be reached.
In this particular situation, it would not seem wise to base a prioritization of the rerouting airports on a criterion linked only to the distance between a position of the aircraft and airport positions, for example, a quantity of fuel needed to reach the rerouting airport, or a total descent duration of the aircraft to the rerouting airport. In practice, a prioritization of this type causes the airports closest to the aircraft to be prioritized, disregarding in particular the airports belonging to the company's network, which are slightly more distant, but are still accessible within the same flight duration.